1. Field of the Invention
The present invention relates to a reticle inspection apparatus for detecting defects on a reticle and, in particular, the present invention relates to a reticle inspection apparatus including an auto-focusing device and having a self diagnosis function for diagnosing the reticle inspection apparatus on error occurring in the auto-focusing device and an automatic correction function for automatically correcting the error of the auto-focusing device on the basis of the diagnosis.
2. Description of the Prior Art
It has been usual that a conventional reticle inspection apparatus for detecting defects on a reticle by irradiating the reticle with laser light includes an auto-focusing device. The auto-focusing device is used to make a focus point of the laser light coincident with a detecting position of the reticle by measuring a distance between an objective lens of an auto-focusing optical system of the reticle inspection apparatus and a detecting surface of the reticle and controlling a position of the objective lens such that the distance between the objective lens and the detecting surface is maintained optimally. In general, such auto-focusing device is operated according to the astigmatic method or the knife-edge method in which a detecting light for detecting defects of the reticle is used in an auto-focusing purpose or the oblique-incidence method in which a light source dedicated to the auto-focusing is used, etc.
Such auto-focusing device has been used in various devices such as cameras, microscopes and semiconductor exposing devices, etc. In the reticle inspection apparatus, the regulation accuracy of auto-focusing operation in the level of 0.1 μm is required due to the increased NA (Numerical Aperture) of a detecting optical system of the auto-focusing device by the recent miniaturization of semiconductor device.
In order to accommodate with this request, U.S. Pat. No. 6,052,478 proposes an auto-focusing device including two optical systems and operating according to the astigmatic method. In the proposed auto-focusing device, an inspection light reflected from an object to be inspected is divided to two inspection light portions, which are incident on the respective optical systems, and the auto-focusing operation is regulated by comparing configurations of beam spots in the respective optical systems. Further, JP H10-030988A and JP H11-306554A propose auto-focusing devices operating according to the oblique-incidence method, respectively.
Among these prior arts methods, the oblique-incidence method will be described. FIG. 1 illustrates a conventional auto-focusing device operating according the oblique-incidence method. As shown in FIG. 1, a conventional reticle inspection apparatus includes a detecting optical system 10, an auto-focusing optical system 30b and a controller system 40b. Incidentally, in FIG. 1, a lateral direction on the drawing sheet is set to an X-direction (rightward being +X and leftward being −X), a vertical direction is set to a Z-direction (upward being +Z and downward being −Z) and a normal direction to the drawing sheet is set to a Y-direction (going direction being +Y and coming direction being −Y).
In the detecting optical system 10, an inspection laser light source 11 emitting inspection laser light 110 to direction −X is provided and, along an optical path of the inspection laser light 110, a telescope 12 for expanding the laser light 110, a mirror 13 for reflecting the laser light 110 to direction −Z, an objective lens 14 for condensing the laser light 110 reflected by the mirror 13 and an X-Y stage 17 mounting a reticle 16 to be inspected and moving the reticle in the X and Y directions are provided in the order.
Subsequent to the reticle 16, a lens 18 for condensing an inspection light 111, which is the inspection laser light 110 transmitted through the reticle 16, and an inspection light sensor 19 for measuring intensity of the inspection laser light 111 condensed by the lens 18 are provided. Further, a beam splitter 20 is provided between the mirror 13 and the objective lens 14, for transmitting the inspection laser light 111 reflected by the mirror 13 to the reticle 16 through the objective lens 14 and deflecting the inspection laser light 111 reflected by the reticle 16 as an inspection laser light 112. Subsequent to the beam splitter 20, a lens 21 for condensing the inspection laser light 112 deflected by the beam splitter 20 and a light sensor 22 for measuring intensity of the inspection laser light 112 condensed by the lens 21 are provided. The objective lens 14 is supported by a regulation mechanism 15. The regulation mechanism 15 regulates a distance between the objective lens 14 and the reticle 16 by regulating height or level of the objective lens 14 with respect to the reticle 16.
In the auto-focusing optical system 30b, a He—Ne laser light source 31 for emitting auto-focusing laser light 310 in direction −X is provided and, subsequent to the He—Ne laser light source 31, a mirror 32 for reflecting the auto-focusing laser light 310 is provided. The mirror 32 is arranged such that the auto-focusing laser light 310 is reflected thereby to direction −Z, travels along a path, which is in parallel to an optical axis 140 of the objective lens 14 and is separated from the optical axis 140, and then is incident obliquely to the reticle 16 through the objective lens 14. Further, a position sensor 33 for detecting an incident position of an auto-focusing laser light 311, which is the auto-focusing laser light 310 incident on and reflected by the reticle 16, is provided.
The controller system 40b includes an image data generator 42 inputted with output signals of the inspection light sensor 19 and the inspection light sensor 22 and coordinates data of the X-Y table 17, for generating an image data of a pattern of the reticle 16 on the basis of measured data of light intensities of the transmitted inspection light 111 and the reflected inspection light 112, a defect extractor 43 responsive to the image data from the image data generator 42, for extracting defect of the reticle 16 on the basis of the image data, an X-Y stage controller 46 connected to the image data generator 42 and the X-Y table 17 for controlling an operation of the X-Y table 17 and outputting the coordinates data of the X-Y table 17 to the image data generator 42, an auto-focusing (AF) controller 48 for arithmetically operating a signal from the position sensor 33 and a lens position controller 49 for regulating a position of the objective lens 14 by actuating the regulation mechanism 15 on the basis of a signal inputted from the auto-focusing controller 48. In addition thereto, the controller system 40b includes an input/output device 41 connected to the image data generator 42 and the defect extractor 43, for instructing various devices in the controller system 40b and outputting an inspected result is provided. Incidentally, the auto-focusing device in this reticle inspection apparatus is constructed with the auto-focusing optical system 30b, the auto-focusing controller 48 and the lens position controller 49.
An operation of the conventional reticle inspection apparatus shown in FIG. 1 will be described. First, the inspection laser light 110 emitted from the inspection laser light source 11 in direction −X is expanded by the telescope 12 and, after reflected to direction −Z by the mirror 13, is condensed by the objective lens 14 and irradiates the reticle 16 mounted on the X-Y stage 17. In this case, the reticle 16 is irradiated with the inspection laser light 110 having substantially minimum beam spot. By moving the X-Y stage 17 mounting the reticle 16 thereon in X and Y directions according to the signal from the X-Y stage controller 46, the irradiating spot of the inspection laser light 110 on the reticle 16 is moved on the reticle 16 relatively. The X-Y stage controller 46 outputs the coordinates data of the X-Y stage 17 to the image data generator 42. The inspection laser light 111, which is the inspection laser light 110 transmitted through the reticle 16, is condensed on the inspection sensor 19 by the lens 18 and its intensity is measured by the inspection light sensor 19. On the other hand, the inspection laser light 112, which is the inspection laser light 110 reflected by the reticle 16, is deflected to direction −X by the beam splitter 20 and condensed on the inspection light sensor 22 by the lens 21. The inspection light sensor 22 measures intensity thereof.
The image data generator 42 generates the image data of the reticle 16 by processing the intensity signals from the inspection light sensor 19 and the inspection light sensor 22 and the coordinates data from the X-Y stage controller 46 and outputs the image data to the defect extractor 43. The defect extractor 43 detects defect of the reticle 16 by comparing the image data with design data, etc., of the reticle 16.
On the other hand, the auto-focusing laser light 310 emitted by the He—Ne laser 31 in direction −X is reflected by the mirror 32 to direction −Z and passes along the optical path, which is parallel to the optical axis 140 of the objective lens 14 and is separated therefrom. Thereafter, the auto-focusing laser beam 310 is transmitted through the mirror 13 and the beam splitter 20, refracted at the objective lens 14 and incident on the reticle 16 obliquely. The auto-focusing laser light 310 is reflected by the reticle 16 as the reflected auto-focusing light 311. The reflected auto-focusing light 311 is refracted by the objective lens 14 again to direction +Z, transmitted through the beam splitter 20 and the mirror 13 and incident on the position sensor 33. The position sensor 33 detects the incident position of the reflected auto-focusing light 311.
In the reticle inspection apparatus shown in FIG. 1, a relation between the incident position of the reflected auto-focusing light 311 on the position sensor 33 and height of the objective lens 14 is determined simply. That is, when height or level of the objective lens 14 is optimal and the inspection laser light 110 is focused on the reticle 16, the auto-focusing light 311 is incident on the position sensor 33 at a predetermined position thereof. On the contrary, when the position of the reticle 16 is sifted in direction +Z or the position of the objective lens 14 is shifted in direction −Z, that is, when the positions of the reticle 16 and/or the objective lens 14 is shifted in mutually approaching direction, the incident position of the reflected auto-focusing light 311 on the position sensor 33 is shifted in direction +X. When the position of the reticle 16 is shifted in direction −Z or the position of the objective lens 14 is shifted in direction +Z, that is, the reticle 16 and/or the objective lens 14 is shifted in mutually separating direction, the incident position of the reflected auto-focusing light 311 on the position sensor 33 is shifted in direction −X.
A signal outputted by the position sensor 33 and indicative of the incident position of the reflected auto-focusing light 311 on the position sensor 33 is processed in the auto-focusing controller 48 to generate a signal necessary to remove the shift. The latter signal is outputted to the lens position controller 49 for regulating the height of the objective lens. The lens position controller 49 operates the regulation mechanism 15 according to the input signal to regulate the height or level of the objective lens 14 such that the incident position of the reflected auto-focusing light 311 on the position sensor 33 becomes the predetermined position. Therefore, the objective lens 14 is regulated such that the distance between the objective lens 14 and the reticle 16 is always kept constant. As a result, the spot diameter of the inspection laser light 110 becomes always minimum in the vicinity of the inspection position of the reticle 16.
However, there are problems in the described conventional technique, which will be described. There may be a case where, when the reticle inspection apparatus is used for a long period of time, the inspection apparatus itself is gradually deformed due to temperature variation and/or atmospheric pressure variation in an environment in which the inspection apparatus is installed. Further, gradual deformation of a structure of the inspection apparatus may also occur by the weight of the inspection apparatus itself. With such deformation of the reticle inspection apparatus, the positional relation between the inspection optical system 10 and the auto-focusing optical system 30b, shown in FIG. 1, and positional relations between the respective constructive components of the inspection optical system 10 and the auto-focusing optical system 30b are gradually changed from the initially set relations. As a result, the auto-focusing function of the reticle inspection apparatus is gradually degraded, so that there is provided an off-focus problem of the image data of the reticle and the reticle inspection cannot be performed normally.
In the case where the inspection of the reticle 16 cannot be performed, it is necessary to judge whether or not there is any error in the auto-focusing function of the reticle inspection apparatus. In order to perform such judgement, it is necessary to check the reticle inspection apparatus itself in detail. Therefore, if the reticle inspection apparatus is installed in a factory on the side of a user, there is another problem that engineers of a manufacturer of the reticle inspection apparatus must visit the user factory and perform the detailed checks of the reticle inspection apparatus therein. This is troublesome and time consuming.